In the manufacture of large structural assemblies, such as for instance aircraft wings, sheets of material, such as aluminum, are fastened together with rivets and/or lockbolt/collars. Rivets are a one-piece fastener, generally have a pin-like body, with a head on one end, and are generally 1/8+L to 1/2+L inch in diameter. In the fastening operation, rivets are inserted into openings which are drilled through the two or more pieces of material (the workpiece) which are to be secured together. The rivet is then upset, using conventional techniques. Prior to insertion of the rivet in the workpiece opening, the rivet is moved from a local storage bin to a gripping element, which typically comprises one or more passive, finger-like elements which are typically part of a ram assembly which inserts the rivet in the opening in the workpiece.
Once the rivet is inserted into the opening in the workpiece, the ram elements on opposite sides of the workpiece are moved into contact with the rivet; great force is then applied against the rivet by the ram, causing upset of the rivet and producing an interference fit with the workpiece. A secure attachment of the material pieces comprising the workpiece results. While rivets come in several different sizes and configurations, the basic installation procedure and the result is the same.
The lockbolt/collar fastening system, on the other hand, is a two-part fastener, combining a lockbolt with a head portion and a pin body having threads on the free end (tail), with a mating collar. The pin tail of the lockbolt is inserted through an opening in the workpiece and the collar is then swaged onto the extending tail of the bolt. Prior to swaging, a collar is moved from local storage to a gripping assembly comprising passive fingers, similar to that for the rivets. In swaging, a ram squeezes on the collar and compresses it tightly over the threads on the pin tail portion of the lockbolt. The combination of the lockbolt and the swaged collar holds the workpiece elements securely together.
In prior art machines used for large-scale assembly operations, spring-loaded, passive fingers are used to hold rivets and/or collars while they are moved to the point of use at the workpiece. The rivet is typically either forced onto the spring-loaded fingers from the back side by means of compressed air or pressed onto the front of the fingers. The rivet then is carried into the hole on the fingers. As the rivet ram pushes the rivet into the opening, the fingers slide back away from the rivet, exposing the rivet for action of the ram in its upset action on the rivet.
In one particular arrangement, the spring-loaded fingers are mounted on the ram and slide along the ram itself. In this arrangement, an inserter device is used to position the rivet onto the finger elements. In another arrangement, the ram itself pushes the rivet partially through the spaced spring-loaded fingers which in turn center the rivet in the opening in the workpiece. The fingers separate from the rivet in both cases by the forcible action of the ram moving the rivet into the opening. However, there is no positive control in gripping and/or releasing the rivet with existing finger arrangements. Instead, the action is passive, with the rivet being moved onto the fingers and then stripped off the fingers. The fingers are not positively controlled.
With such an arrangement, the rivet can become slightly misadjusted and cause damage to the workpiece as the rivet is inserted into the opening. Further, with such an arrangement, there is no opportunity for definitive confirmation that the rivet is of the proper size and configuration, and still further, whether or not there is even a rivet at all present on the fingers to be inserted.
Similarly, prior art collar gripping arrangements also include spring-loaded fingers on which the collar is positioned. Such collar positioning systems also typically use a centering pin, as shown in U.S. Pat. No. 5,437,094, which includes an O-ring arrangement. An enhancement of such an arrangement includes the use of a split metal ball instead of the O-ring. Generally, however, in all these collar-insertion systems, the collar must be stripped away from the spring fingers. Like the rivet fingers, there is no positive control over the action of the fingers and hence no positive control over positioning of the rivet or collar relative to the fingers. In both cases, the collar must be forcibly removed from the fingers. Separate powered devices must be used to move the rivet/collar onto the fingers and to remove the rivet/collar therefrom. Such a collar holding arrangement results in placement accuracy problems and a lack of required precision in reliably placing the collar on the exposed end of the lockbolt tail. In some cases, improper swaging of the collar results.
While relatively few errors generally occur with these prior systems, the errors which do occur are no longer satisfactory; higher standards of accuracy and performance are being implemented. Furthermore, errors produced by prior systems for holding rivets and lockbolts have occasionally resulted in damage to the workpiece, which is unacceptable.
It is desirable that gripping systems for rivets and collars used in large-scale assembly operations be fast, efficient and extremely reliable, resulting in very few, ideally no, errors. Preferably, it would be desirable that such a system have the capability of positively identifying the presence of a collar or rivet in the respective gripping systems, and further, that the rivet is the correct size and configuration for the particular opening in the workpiece.